Chapter 10: Life on Earth

Introduction

The only known life in the universe. So far.

It's possible, maybe even likely, that there is life elsewhere in the universe, but if there is life we haven't found it yet. What makes Earth unique in the solar system is not only that it has life but that it has such an incredible diversity of life forms. The origin of life, the evolution of a variety of life forms, and the evolution of life that can read this sentence is certainly unique in the universe.

Chapter Summary

Summary

Although different ecosystems differ greatly from one another, the structure of an ecosystem is the same. There must be a source of food energy, usually from photosynthesis, and then herbivores, predators, scavengers, and decomposers, among others. Energy flows through the ecosystems in tropic levels and connections between organisms are made in a web, known as the food web. There are many types of ecosystems in fresh water, the oceans, and on land. Organisms must be well adapted to their habitats or they may go extinct. Extinction of a species opens up a niche, which a different species will likely evolve to fill. This has occurred throughout Earth's history as mass extinctions have opened habitats and adaptive radiation has acted on a different set of organisms to fill those habitats. The earliest life was simple cells, possible with RNA as the nucleic acid. Photosynthesis evolved and provided a food source for the food web, plus oxygen to the atmosphere. Multicellular life didn't evolve for 4 billion years. During most of the Paleozoic, life was restricted to the seas. Reptiles ruled in the Mesozoic and even in the Cenozoic life was fairly different from what we see today. The biological processes that govern the evolution of species has resulted in tremendous biodiversity we see today. This includes the evolution of humans, which is better understood as more fossils are discovered.